Projection welding of an aluminum sheet

ABSTRACT

A projection coined onto a metal sheet, preferably an aluminum sheet, for projection welding the sheet to an adjacent metal sheet. The projection has a thickness greater than the thickness of the sheet on which it is formed. The wall of the projection surrounds a first recess and a second recess is formed in a side of the sheet opposite the projection from which the metal cold flows to form the projection. A welding gun assembly having an improved low interia, fast response to the collapse of the projection welding the sheet having the projection to the adjacent sheet.

FIELD OF THE INVENTION

[0001] This invention relates to projection welding of metal sheet toanother metal body and, more specifically, to an improved projection andprojection forming process for thin aluminum sheet for projectionwelding thereof, and an improved welding gun for use therewith.

BACKGROUND OF THE INVENTION

[0002] Projection welding is a known technique for joining twooverlapping metal sheets. In conventional projection welding, a smallprojection is provided on one of the sheets and extends transversely toa side of the sheet so that the tip thereof contacts the other sheet. Anelectrode of a welding gun is pressed into contact with one of thesheets in alignment with the projection, and a combination of force andwelding current is applied to the electrode to cause the projection tocollapse and form a weld nugget which joins the two sheets together atthe contact area defined by the projection.

[0003]FIG. 1 illustrates a known arrangement 10 for projection hemwelding. This welding arrangement 10 is particularly desirable forprojection welding of thin sheet-like metal components disposed inoverlapping relation, typically light-gauge steel sheets which commonlyhave a thickness in the range of about 0.020 to about 0.050 inch. Thethin sheets 11 and 12 have portions which directly overlap, and in a hemwelding process one of the sheets 11 has a flange or hem part 13 whichis bent to overlap another side of the other sheet 12. The sheet 12 issandwiched between the sheet 11 and its hem part 13. Intermediate sheet12 has a bead or projection 14 projecting transversely therefrom so thatthe tip of the projection contacts the adjacent surface of the hem part13. A projection weld is created Directly between the sheet 12 and thehem part 13 at the region of the projection 14.

[0004] An adhesive 16 can be placed between the two sheets 11, 12 (FIGS.2 and 3). The adhesive 16 is a heat curable adhesive that will furthersecure the two metal sheets together after it is activated by applyingheat thereto, usually in an oven.

[0005] In the projection welding arrangement 10 as shown in FIG. 1, theoverlapping sheets 11, 12 are typically positioned on a support or die15, and a movable welding head assembly 17 is positioned adjacent thedie 15 to effect the projection weld between the overlapped sheets. Thewelding head assembly 17 includes a hollow housing 18 having anelectrode 19 movably supported therein and projecting outwardly forcontact with the overlapped sheets in the region of the projection, anda spring 21 is confined within the housing 18 and acts against an innerface of the electrode 19 so as to urge the electrode outwardly into anextended position, in which position the electrode abuts an interiorstop surface formed on the housing. The welding head assembly 17 iselectrically connected to a stationary transformer 22. The power supplyto the transformer 22, and hence the welding current supplied to thewelding head assembly, is controlled by a suitable control unit 23. Thetransformer 22 has the primary coils 24 thereof connected to suitableelectrical conductors 25 and 26 that supply electrical energy to thetransformer 22. The secondary coils 27 of the transformer are in turnconnected to electrical conductors 28 and 29, one of which is connectedto the welding head assembly 17 and the other is connected to theworkpieces 11, 12, such as being connected either to the die 15 or to anelectrical contact gun 31. The contact gun 31 includes a support 32 suchas a conventional double-acting pressure cylinder, normally an aircylinder, having a conventional electrode 33 movably supported thereon.The electrode 33 engages a part 34 of the intermediate sheet 12, whichpart 34 is shown as spaced from the overlapping portions of the sheets.The conductors 28 and 29 are typically constructed of a conventionalflexible lamination so as to permit respective movement of the weldinghead assembly 17 and contact assembly 31 relative to transformer 22.

[0006] The welding head arrangement also includes a drive device 36 foreffecting movement of the welding head assembly 17. The drive deviceconventionally comprises a pneumatic cylinder 37 having a housing 38,which is typically stationarily mounted spaced and separate from thewelding head assembly 17. An extendible and contractible piston rod 39extends from the housing 38 and couples the housing 18 of the weldinghead assembly 17 to the drive device 36. The piston rod 39 controls themovement of the assembly 17 into engagement with the sheets 11, 12 whena projection welding operation is carried out.

[0007] When projection welding as summarized above, the projection istypically stamped or embossed on the thin steel sheet by opposed dieswhich deform the sheet by forcing a portion thereof sidewardly so as todefine a projection which projects sidewardly of the sheet generally inthe direction of the force applied by the forming die. The projectiontypically comprises a geometric shape such as a truncated conical orpartial spherical shape as it projects transversely from the sheet. Thismethod of forming the projection and the resulting shape thereofnecessarily results in the wall thickness of the projection beingthinner than the thickness of the base sheet, and also typically resultsin the base wall of the projection (i.e. the portion of the wall wherethe projection joins to the base sheet) being disposed in a sloped orangled relationship relative to the plane of the sheet. These latterconfigurational features, however, have not detrimentally effected theability of the projection to create proper quality welds between thinsteel sheets since steel possess a high tensile strength and thus isable to withstand the significant compressive force applied theretoprior to reaching the actual melting or welding temperature. Prematurecollapse of the projection during projection welding of sheet steel hasthus not presented a significant problem.

[0008] When projection welding an aluminum sheet, however, totallydifferent melting temperature and tensile strength properties areexhibited by aluminum sheet in comparison to steel sheet, andaccordingly repeatably and successfully effecting projection welding ofaluminum sheets can not normally be achieved. More specifically, notonly does aluminum possess a significantly lower strength than steel,but more significantly it has been observed that the yield strength ofaluminum undergoes a significant decrease when aluminum is heated to atemperature between two and four hundred degrees F. (FIG. 4), and infact this significant decrease in strength occurs over a very smalltemperature range which is still significantly below the melting orwelding temperature for aluminum. The many prior attempts to projectionweld thin aluminum sheets have hence mostly met with failure since theprojections have exhibited premature collapse thereof at a temperaturewhich is significantly below welding temperature. Thus, it has not beenrepeatably possible to properly maintain the requisite electrodepressure on the projection, nor has it been repeatably possible for theelectrode to properly follow up the collapsing of the projection so asto maintain proper current-transmitting contact therewith. The propercontact and hence transfer of current to the projection, and the properconcentration of the current through the small electric contact areadefined by the projection, have thus not been dependably and repeatablyachievable, and accordingly proper weld nuggets have not typically beenachievable when attempting to projection weld aluminum sheets.

[0009] Accordingly, it is an object of this invention to provideimprovements for projection welding of thin metal sheets andparticularly improvements applicable for permitting successfulprojection welding in situations where the thin metal sheet having theprojection formed thereon is constructed of aluminum.

[0010] More specifically, it is an object of the invention to provide animproved projection which is monolithically associated with a thinaluminum sheet for permitting successful performance of a projectionwelding operation, which projection due to its improved shape, size andforming process provides significantly increased strength so as towithstand the electrode pressure imposed thereon without experiencingpremature collapse, thereby maintaining both proper interface pressure,contact area, and current density between the projection and theadjacent metal body until reaching the temperature at which theprojection rapidly collapses and permits creation of a metallurgicalbond between the overlapped metal sheets and body.

[0011] In the present invention, the projection is formed generally as ahollow upright cylindrical wall which is cantilevered perpendicularlyaway from one side of the aluminum sheet. The hollow upright wall has awall thickness which approaches or is substantially equal the thicknessof the sheet, and also has significant height so as to define a columnhaving significant strength against compression. The column where itjoins the sheet is also backed by a full thickness of the sheet. Theinterior of the column opens downwardly from the free end thereof so asto terminate at a bottom wall which, in a preferred embodiment, issubstantially flush with the upper surface of the sheet but is ofsignificantly reduced thickness in comparison to the sheet thickness. Aforming recess opens inwardly from the opposite side of the sheet incoaxial alignment with the projection and terminates at the thin bottomwall. The forming recess typically is limited by an outer side wallwhich does not extend radially beyond the radial extent of the innerwall of the hollow upright cylindrical wall.

[0012] The projection of this invention is preferably formed by acoining operation whereby the sheet material is squeezed sidewardlyduring forming of the bottom recess and is then forced to flow outwardly(i.e. perpendicularly) so as to permit creation of the hollow uprightcylindrical wall. The material forming the upright thus undergoessignificant cold working which not only effects creation of the thickupright wall so as to provide significant column strength, but alsoeffects significant cold working of the material so as to furtherincrease the strength thereof.

[0013] The present invention also includes an improved welding gunwhich, in conjunction with the improved projection, permits performanceof successful projection welding of thin metal sheets, particularlyaluminum sheets. The welding gun employs a spring-urged electrode whichpossesses minimal mass and inertia so as to readily respond to thecollapse of the upright projection during the welding operation, therebymaintaining proper contact pressure and concentration of the weldingcurrent to thus result in a desired weld nugget or joint.

[0014] Other objects and purposes of the invention will be apparent topersons familiar with technology of this general type upon reading thefollowing specification and inspecting the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic view of a conventional projection weldingsystem.

[0016]FIG. 2 is an enlarged view of conventional thin metal sheets forprojection welding.

[0017]FIG. 3 is an view similar to FIG. 2 showing the collapsedprojection and creation of a weld nugget.

[0018]FIG. 4 is a graph of yield strength versus temperature for steeland aluminum.

[0019]FIG. 5 is a plan view of the projection of the present invention.

[0020]FIG. 6 is a cross sectional view taken along line 6-6 in FIG. 5.

[0021]FIG. 7 is a cross sectioned view of a die arrangement for formingthe projection.

[0022]FIG. 8 is an elevational view of an improved welding gun.

[0023]FIG. 9 is a longitudinal cross sectional view of the gun of FIG.8.

[0024]FIG. 10 is an enlarge partial cross-sectional view of a part ofFIG. 9.

[0025] Certain terminology will be used in the following description forconvenience in reference only, and will not be limiting. For example,the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” willrefer to directions in the drawings to which reference is made. Thewords “inwardly” and “outwardly” will also refer to directions towardand away from, respectively, the geometric center of the arrangementbeing described and designated parts thereof. Such terminology willinclude the words specifically mentioned, derivatives thereof, and wordsof similar meaning.

DETAILED DESCRIPTION

[0026] The following description of a preferred embodiment of thepresent invention will first describe the structure of the projection,then describe the dies and method for creating the projection, andthereafter describe the welding gun used to create a projection weldbetween a metal sheet and adjacent metal body, at least one of which ispreferably of aluminum.

[0027] Referring to FIGS. 5 and 6, there is illustrated an improvedprojection 51 according to the present invention, particularly forprojection welding of relatively thin metal sheets. The projection 51 isintegrally and monolithically formed as a part of a substantially largebut thin metal, and preferably aluminum or aluminum alloy, sheet 52which has substantially planar and parallel first and second sidesurfaces 53 and 54, the latter herein being referred to as upper andlower surfaces for convenience in reference. The term “sheet” refers toan metal material, preferably aluminum or aluminum alloy, having athickness of about 0.006 to 0.249 inch.

[0028] The projection 51 is defined primarily by an endless, here shownas annular, upright wall 55 which is preferably tubular in cross sectionand is formed about an axis 56 which extends substantiallyperpendicularly with respect to the plane of the sheet 52. The uprightwall 55 is thus cantilevered upwardly from the sheet 52 in substantiallyperpendicular relationship therewith and forms a cylinder.

[0029] The cylindrical wall 55 is defined by concentric outer and innerannular side surfaces 57 and 58, respectively, which ideally formaxially aligned, elongate cylindrical surfaces of uniform diameter.However, due to the constraints of manufacturing the projection, thecylindrical side surfaces 57 and 58 normally are slightly inclinedrelative to the vertical so that these surfaces 57 and 58 thus slightlysymmetrically converge toward one another and toward the vertical (e.g.axis 56) as they project axially toward the free end of the upright wall55. This thus provides the upright wall 55 with a very slight inner andouter taper as defined by the respective inner and other, cylindricalside walls 57, 58 to facilitate forming of the projection by dies, andalso the separation of the dies after forming of the projection. Theslight taper of these side walls is maintained at a minimum small angle,such as about 2° from the perpendicular to the upper sheet surface 53.The free end of the upright wall 55 is defined by an end wall 59 which,in cross section, is of an arcuate convex configuration resembling asemicircle for connection between the generally concentric outer andinner side walls 57 and 58.

[0030] The upright wall 55 defines therein a recess 61 which, due to itsbeing bounded by the inner cylindrical side wall 58, is substantiallycylindrical in configuration and opens coaxially outwardly through thefree end 59 of the projection. This recess 61 is a blind opening in thatthe inner or lower end thereof is closed off by a base wall or web 62which extends transversely across the bottom of the recess. This basewall 62 in turn separates the projection-defining recess 61 from afurther generally cylindrical recess 63 which opens upwardly into thethickness of the sheet 52 from the bottom surface 54 thereof. Recess 63is also centered about the axis 56 so as to be substantially coaxiallyaligned with the recess 61. The recess 63 is defined by a surroundingcylindrical wall 64 which, while preferably extends perpendicularly tothe bottom surface 54, is slightly inclined relative to theperpendicular as it extends vertically so as to facilitate manufactureof the projection, e.g. the wall 64 may be inclined about two degrees.The cylindrical wall 64 has an average diameter which is preferablydimensionally similar to (i.e., between about 75% and 125% of) theaverage diameter Di of the inner cylindrical wall 58 defining the recess61. The wall or web 62 extends transversely and closes off both therecesses 61, 63 and effects separation thereof. This web 62 ispreferably of small thickness in comparison to either the thickness “t”of the sheet 52 or in comparison to the height “h” of the wall 55.Further, the base wall 62 is preferably positioned so that it extendstransversely across and closes off the projection recess 61 at alocation spaced downwardly a substantial distance from the upper freeend 59 thereof and, in the illustrated and preferred embodiment, the web62 is positioned such that the upper surface 65 thereof is substantiallycoplanar with the upper surface 53 of the sheet 52 such that the web isrecessed downwardly of the apex of the free end 59 a distance “h”.

[0031] The projection 51, where it joins to the sheet 52, is preferablysurrounded by an annular groove or moat 66 which is depressed downwardlyinto the thickness of the sheet 52 from the upper surface 53 thereof.This surrounding moat or groove 66 preferably has the inner annularboundary wall 67 thereof formed substantially as a downward extension ofthe outer cylindrical side wall 57, with this boundary wall 67 smoothlymerging into a bottom wall of the moat, the latter preferably being of arounded concave cross section. The moat 66 has a depth which enables itto project downwardly into the thickness of the sheet 52, with the depthof the moat typically being in the range of from 20 percent to 65percent of the sheet thickness.

[0032] Due to the provision of the recess 63 formed on the bottom sideof the sheet in coaxial alignment with the projection recess 61, and theadditional provision of the moat 66 which exteriorly surrounds theprojection 51 and opens downwardly into the sheet from the top surface53 thereof, there is thus defined an annular wall portion 69 which ispart of the sheet 52 and is defined generally radially between therecess 63 and the concentrically surrounding moat 66. This annular wallportion 69 effectively constitutes an axial extension of the annularupright wall 55.

[0033] As to the relative sizes or proportions of the projection 51relative to the sheet 52, the projection preferably has a height “h” asmeasured perpendicularly between the free end 59 and the upper surface53, which height “h” is preferably greater than 0.5 times the thickness“t” of the sheet 52. In one embodiment of the projection 51, the height“h” is less than about the thickness “t”. The upright wall 55 alsopreferably has a wall thickness (i.e., one-half the difference betweenthe average outside diameter Do and the average inside diameter Di)which is of similar magnitude to the sheet thickness “t” i.e., theaverage radial thickness of wall 55 is preferably at least 0.75 timesthe sheet thickness “t”. In an embodiment of the present invention, theaverage radial thickness of the wall 55 generally equals the sheetthickness “t”. Further, the cylindrical projection 51 has a averagediameter Dm which is about 2½ to about 5 times the sheet thickness “t”,and preferably about 5 times the square root of thickness “t”. However,the diameter Dm may be increased to increase the strength of the weldbetween the sheets by increasing the joined area. Also, the thickness ofthe base wall or web 62 is typically less than 0.5 times the sheetthickness “t” and more preferably is no more than about 0.25 times thesheet thickness “t”. And, in a preferred embodiment, the depths of therecess 63 and moat 66 preferably are such that the bottom or lowermostpoint of the moat 66 is located substantially at or below a horizontalplane defined by the uppermost point (i.e., the upper closed end) of thebottom recess 63, that is the moat depth generally equals the thicknessof the web 62.

[0034] The projection 51 is formed on the thin sheet 52 by opposed dieswhich are shaped to effect pressing of the sheet therebetween so as toresult in suitable cold working of the sheet to effect forming of theprojection. The preferred forming process is known as “coining”, andutilizes opposed dies which are simultaneously moveable in a singleoperation so as to effect pressing and squeezing of the sheettherebetween to effect the desired cold working of the sheet. Exemplarycoining dies 71 and 72 are shown in FIG. 7. The dies 71 and 72 aremovably positioned on opposite sides of the sheet 52, which sheet issecurely held in a suitable holding or clamping structure (not shown),with the opposed dies 71 and 72 then being simultaneously moved inwardlytoward one another to engage the sheet therebetween and effect formationof the projection.

[0035] The dies 71 and 72 illustrated in FIG. 7 are herein referred toas female and male dies, respectively, inasmuch as the upper die 71defines thereon the configuration which results in formation of theprojection 51, whereas the lower die 72 defines thereon a configurationwhich results in formation of the bottom recess 63.

[0036] More specifically, the female die 71 has an annular rib or dam 73which projects perpendicularly outwardly from the flat front face 74.The annular dam 73 and its projection outwardly from the face 74 is suchso as to result in formation of the moat 66 during the projectionforming process. The die 71 also has an endless channel 75 which isdisposed radially directly inside the dam 73 and which is recessedperpendicularly inwardly into the depth of the die beyond the front face74. The channel 75 has a size and configuration which corresponds to andhence results in formation of the upright wall 55 as described above.The channel 75 surrounds a center hub 76 which has an end surface 77thereof disposed substantially coplanar with the end face 74, which hub76 thus defines the configuration of cylindrical recess 61 definedwithin the projection.

[0037] The opposed male die 72 defines thereon a flat front face 78which is disposed in generally parallel relationship to the front face74 of the female die 71. A forming projection or anvil 79 of generallycircular cross section projects perpendicularly outwardly from the face78 and has a configuration which corresponds to the bottom recess 63 soas to permit formation thereof.

[0038] The dies 71 and 72 are disposed in opposed relationship so thatthe channel 75 and anvil 79 are coaxially aligned along a common axis 81and are normally spaced a sufficient distance apart so as to permit asheet 52 to be positioned therebetween and clampingly held. The opposeddies 71 and 72 are then moved inwardly, preferably simultaneously, dueto application of driving forces F thereto, the latter being effected bya conventional press mechanism, at a velocity which allows the sheetmetal material, preferably aluminum or aluminum alloys to flow. The dies71 and 72 are simultaneously drivingly moved inwardly so that theannular dam 73 and anvil 79 engage opposite sides of the sheet 52, wherefurther inward driving of the dies causing the dam 73 to penetratedownwardly through the upper surface of the sheet to initiate formationof the moat 66, and at the same time the anvil 79 initiates penetrationupwardly through the bottom surface of the sheet so as to permitinitiation of the formation of the bottom recess 63. This inwardpressing of the dies 71 and 72 toward one another continues until theopposed front faces 74 and 78 are effectively pressingly engaged withthe respective upper and lower side faces of the sheet. When reachingthis position the material of the sheet 52 has been suitably coldworked, e.g. coined, so as to effect formation of the projection 51substantially as illustrated in FIG. 6.

[0039] During the forming of the projection as briefly summarized above,the opposed ends of the anvil 79 and hub 76 cooperate to effectivelysqueeze (i.e., cold work) the aluminum sheet material radially outwardlyinto the annular void defined by the channel 75 and, at the same time,the penetration of the annular rib 73 into the upper surface of thesheet material and the resulting moat 66 defined thereby effectivelyassists in causing the cold flowing material which is squeezed radiallyoutwardly between the opposed center hubs 76 and 79 to be deflectedaxially upwardly so as to flow into and effectively fill the channel 75so as to permit formation of the upright wall 55. During this coining ofthe projection, the cold working of the aluminum sheet material effectssignificant flow of the material sidewardly relative to the forcedirection inasmuch the material which originally occupies the bottomrecess 63 is forced sidewardly and thence axially so as to ultimatelyfill the channel 75. The formation of the projection 51 by this coiningoperation, and the significant cold working of the material during thecoining operation, thus enables a significantly greater quantity ofmaterial to be utilized for forming the upright wall 55 so that the wallcan have desired height and thickness properties, and also enables thewall to be substantially perpendicular to the base sheet material tothus provide optimum strength properties. This cold working of thematerial also significantly increases the strength and hardness of theresulting projection, thereby providing more desirable properties withrespect to its successful utilization for projection welding.

[0040] Further, due to the shape of the projection 51 and the formingprocess associated therewith as summarized above, the annular uprightwall 55 defining the projection 51 effectively extends downwardly to thebottom surface 54 of the sheet since the annular wall portion 69 issubstantially coaxially aligned with and hence effectively constitutesan extension of the upright wall 55, thereby maximizing the uprightcolumn strength of the projection relative to the base sheet material.This annular wall part 69 and its disposition directly under the uprightcolumn thus provides reinforcement for the column and preventsundesirable premature collapse thereof during projection welding.

[0041] In one exemplary embodiment of the invention, the projection 50is formed on a thin aluminum sheet 52 having a thickness in the range ofabout 0.030 to 0.035 inch. The height of the projection wall 55, and thedepth of the female die channel 75 into which the metal flows to createthe wall, equals about 0.028 inch. The diameter of the cylinder definedby the center of projection wall 55, i.e. Dm, is about 0.15 inch. Thewidth of the base of the wall roughly equals the thickness of the sheet52, and thus is in the range of about 0.030 to 0.035 inch. The moat 66has a depth of about 0.20 inch.

[0042] The projection 51 as described above is preferably utilized inconjunction with a low-inertia fast-response welding gun, one example ofwhich is illustrated in U.S. Pat. No. 5 714 730 owned by Newcor, Inc.,one of the Assignees hereof. More specifically, the welding gun for usewith the projection 51 incorporates therein additional features whichare disclosed in copending U.S. Ser. No. 08/895 526, also owned byNewcor, and the disclosure of which is herein incorporated by reference.To further improve on the welding gun for effecting projection weldingof aluminum sheets employing the improved projection 51 described above,specifically to further reduce the mass and inertia of the welding gunand further improve the fast follow-up response of the gun duringcollapse of the projection, the construction and operation of thewelding gun has been further improved to incorporate additionalmodifications which are described below with reference to FIGS. 8-10.

[0043] Referring to FIGS. 8-10, there is illustrated an improvedintegral projection welding head and drive assembly 101 according to thepresent invention, which integrated assembly 101 replaces the drivedevice 36 and head assembly 17 of the conventional system shown inFIG. 1. This assembly 101, which includes an electrode assembly 102mounted on a fluid pressure drive cylinder 103, is particularlydesirable for creation of a projection weld between overlapping thinmetal sheets, such as illustrated in FIG. 1, and in particular forcreation of a projection weld involving the inventive aluminumprojection 51 discussed above. The assembly 101 is also particularlysuitable for application of a short-duration unipolar current pulse or aplurality of reduced amplitude current pulses each having a durationless than one half cycle of an alternating current wave form, and forpreloading the electrode spring force as described in the aforementioned′730 patent. The assembly 101 further includes a low-interia,fast-response force generating unit 104 providing an outward biasingforce to the electrode assembly 102, and an adjusting structure 106 forselecting the force generated by the force generating unit 104.

[0044] Specifically, the drive cylinder 103 has an elongate housing 107including an elongate cylinder wall 109 surrounding a bore 111. The bore111 is respectively closed at its upper and lower ends by upper andlower heads 113, 114. As a result, the bore 111 defines a fluid drivenchamber in which a piston 116 is reciprocally driven downward and upwardeffecting movement of the electrode assembly 102. More specifically, thepiston 116 is slidably received in the bore 111 and fluidly divides thebore into a forward pressure chamber 118 and a return pressure chamber119. At least two ports (not shown) respectively communicate fluid tothe pressure chambers 118, 119 to effect movement of the piston 116. Abearing housing or carrier 121 is fixed at one end of the drive cylinder103 adjacent the lower head 114, has a cylindrical bushing 122 fixedtherein, and defines a lower free end 127 remote from the head 114.

[0045] A cylindrical rod 123 is securely cantilevered to the piston 116at a fixed end 124 and extends through the return pressure chamber 119,lower head 114, bearing carrier 121, and is slidably supported inbushing 122 so that a lower free end 126 of the rod 123 extends axiallybeyond the bearing carrier lower end 127. The rod 123 includes a blindbore 129 opening from the rod free end 126 and extending axially alongthe longitudinal central axis 150 substantially the length of the rod123 and ending adjacent at the fixed rod end 124. The bore 129 of rod123 has a stepped configuration wherein a first bore portion 132 ispositioned closest the fixed rod end 124 and has a closed upper end.Second, third and fourth bore portions 133, 134, 135 are sequentiallyarranged from the first bore portion 132 to the free rod end 126. Thediameters of the bore portions 132, 133, 134, 135 respectively increase.The second bore portion 133 receives a cylindrical bearing or bushing137 therein. The fourth bore portion 135 is internally threaded.

[0046] The electrode assembly 102 is spaced from the drive cylinder 103and includes a generally tubular electrically conducting adapter block141 having a coaxial mounting opening 142 and electrode-receivingopening 144 at opposite ends thereof. The mounting opening 142 has adiameter greater than the diameter of the electrode receiving opening142. The adapter block 141 at the mounting opening end is diameterallycut so that the mounting opening 142 of the block closes when a reducedheight split clamp 145 is tightened thereon. A portion of the splitclamp 145 encircles the block 141 and has only a sufficient height toreceive a screw therein perpendicular to the splits in the clamp and theblock so that the screw can tighten the clamp onto the block whilepartially closing the mounting opening 142. The clamp 145 includes areduced height flange 147 which projects sidewardly (i.e., radially)relative to the axis or movement direction 150, and has a threadaperture 148 extending vertically there through parallel to the axis 50.The clamp 145 is configured to provide a minimal mass. The electrodereceiving opening 144 receives a bolt-like electrode 140 therein. Theelectrode 140 may, as illustrated, include a threaded elongate stem andan enlarged lower workpiece-contact end that is integral with one end ofthe stem and contactingly adjacent the adapter block 141 when theelectrode is fully threaded into the adapter block.

[0047] An electrical power supply assembly 149 supplies electricalcurrent to the electrode assembly 102 and is secured to a lower portionof the housing 107 adjacent a lower end thereof. The power supplyassembly 149 includes a holder or clamp ring 151 received over and fixedto the carrier 121, for example by set screws (not shown). A radiallyextended flange 153 of the holder 151 is declined about 20 degrees withrespect to the remainder of the holder 151 and includes at least one,and preferably two, bolt receiving apertures 154 extendingperpendicularly therethrough. An electrical conductor 158, typically alaminated flexible conductive shunt defined by a plurality ofsuperimposed thin copper plates, is electrically connected at one end toan electrical power supply, for example the transformer 22, control 23and voltage source shown in FIG. 1, and at the other end to one end of aflexible electrical conductor 159, with these adjacent ends beingsecured to the flange 153. The conductor 159 is a plurality of elongateelectrically conductive flexible plates, e.g. superimposed thin copperplates, fixed together at each end by metal caps or clips 161, e.g.silver plated copper C-shaped clips. The conductors 158, 159 each havean aperture therethrough adjacent the ends thereof through which a bolt160 extends to secure the conductor and lamination together onto theflange 153. Insulative sleeves 156 are respectively positioned in theapertures 154 and insulative washers 157 are positioned on respectiveupper and lower faces of the holder portion 153 so as to electricallyinsulate the holder 151, and hence the carrier 121 and housing 107, fromthe electrical conductors 158, 159. The other end of conductor 159 issecured, eg. bolted, to the flange 147 of the clamp 145 to transmitelectrical energy therethrough to the adapter block 141 and electrode148. The conductor 159 extends a short distance from the flange 153 tothe clamp 145 in a generally C-shape with its lower leg 162 extendingessentially perpendicular to the axis 150 in the fully extended positionof the electrode assembly 102 as shown in FIGS. 9 and 10.

[0048] The bore 139 of the rod 123 slidably receives an elongate hollowrigid tube 170 centered about the central axis 150 and which ispreferably a cylindrical tube made of a chromolly. The bushing 137slidably journals the upper part of the tube 170 therein. The tube 170has an outer diameter at least slightly less than the diameter of thefirst bore portion 132 so that it is readily receivable therein. Thesmooth-walled tube 170 is partially housed within the fourth boreportion 135 and extends axially outwardly beyond the bore 129 and thefree rod end 126. The electrode assembly 102 is fixedly mounted at alower longitudinal end of the hollow tube 170 and is spaced from the rod123 The lower end of the tube 170 is received in the mounting opening142. The clamp 145 is tightened on the split portion of the adapterblock 141 which fixes the clamp 143 and the block 141 onto the free endof the tube 170.

[0049] The force generation unit 104 includes an axially elongate spring175, preferably a coil spring, that is entirely housed in the bore 129,and entirely in the third bore portion 134 in the illustratedembodiment. The spring 175 has a diameter less than the diameter of thethird bore portion 134 and greater than the outer diameter of the tube170 so that the spring 175 is free from frictional interference and iselectrically insulated from the rod 123 or tube 170. A ring-like bearingwasher 176 is mounted on an electrically insulative washer 177 which ismounted on the step face between the second and third bore portions 133,134. The washers 176 supportingly receives an upper end of the spring175 thereagainst on the axially downwardly facing side thereof. Thus,the bearing washer 176 acts as a support surface for the spring 175 andthe insulative washer 177 supports the bearing washer 176. On theaxially upward facing side of the washer 177, an end of the cylindricalbushing 137 rests thereon to assist the holding of the bushing 137 inthe second bore portion 133.

[0050] The adjusting structure 106 includes a stop or end assembly 179that is positioned at the free rod end 126 and includes a generallycylindrical fitting 181 that is externally threaded and is threadedlyreceived in the fourth bore portion 135. The fitting 181 has radiallyoutwardly extending drive flanges 182 at a lower end thereof. The driveflanges 182 are axially spaced from the free rod end 126 so that thedrive flanges are engageable by a suitable means, ie. a tool ormanually, to rotate the end assembly 179 relative to the fourth boreportion 135 thereby axially positioning the end assembly in the fourthbore portion. The fitting 181 also has a cylindrical longitudinalthrough opening 183 extending centered about the longitudinal centralaxis 150. The opening 183 receives an electrically insulative,cylindrical bushing 184 fixed therein that slidably supports the lowerpart of the tube 170. The upper face of the bushing 184 may not axiallyextend above the upper face of the fitting 181. A lightweight, e.g.aluminum, split stop ring or collar 186 is positioned above the upperend of the adjustment fitting 181 in either the third or fourth boreportion 134, 135 intermediate the ends of the tube 170. The collar 186has apertures 185 in each half receiving connecting screws joining thehalves together and has a greater radial wall width than the cylindricalbushing 184 so that it extends radially outwardly relative to thebushing. The collar 186 extends radially outwardly so that a lower face188 thereof contacts the upper end of the fitting 181 stopping thecollar 186 in its lowermost axial position. An upper face 189 of theannular collar 186 abuts a lower end of the spring 175, thus defining asecond support surface for the spring with the spring 175 forcing thecollar 186 downwardly onto the fitting 181.

[0051] An anti-rotation mechanism 190 is positioned intermediate theends of the tube 170 within the third or fourth bore portions 134, 135and includes, an electrically insulative sleeve 193 fixed in the tube170, a pin 191 extending radially from the tube 170 and pressed into thesleeve 193, and an elongate slot 192 extending longitudinally in the rod123. The slot 192 receives the pin 191 therein after the pin passesthrough a radial bore in the collar 186. The slot 192 has a widthsubstantially equal to the diameter of pin 191 to prevent relativerotational movement between the central tube 170 and rod 123. The slot192 has a height significantly greater than the diameter of the pin 191for allowing relative axial movement of the tube 170 with respect to therod 123.

[0052] The assembly 101 is secured to an attachment plate 204 that issecured to a fixed support 206 and axially slidable on the support 206(FIG. 9). A conventional position selection mechanism 207 fixes theattachment plate 204 to the support 206 preventing movement therebetweenonce the assembly is positioned for repeatable welding operations.

[0053] The operation of the integral welding head and drive cylinderassembly 101 of the present invention will now be described inconjunction with the projection 51.

[0054] It is desirable to precisely control and predetermine the forceapplied onto the projection 51 by the electrode assembly 102 during aprojection welding operation by providing a fast response of the weldingassembly 101 onto the projection 51 during the welding operation. Thepresent invention provides for the precompression of the spring 175,which is entirely housed in the rod 123 and biases a reduced mass andinertia tube 170 and electrode assembly 102, so that repeatable anduniform projection welds are created by the inventive projection weldingassembly 101.

[0055] When the electrode assembly 102 is fully assembled and in itsfully extended state as shown in FIG. 10, the electrode assembly islinearly extended from the rod 123 by its greatest extent and theconductor shunt 159 is in an C-shape. In the electrode assembly's fullyextended state, the spring 175 is confined and precompressed betweenaxially spaced support surfaces, namely the bearing plate 176 and collar186, to create the preload force. More specifically, one end of spring175 is fixed relative to the rod 123 by the bearing plate 176. The otherend of the spring 175 is confined by the collar 186, which collar isaxially moveable relative to the rod 123 and bearing plate 176. Theaxial positioning of the first support surface, collar 186, relative tothe second spring support surface, bearing plate 176, is set by theposition of the end assembly 179 in the fourth bore portion 135. The endassembly 179 is shifted in its axial position in the fourth bore portion135 by threading the fitting 181 in the fourth bore portion to axiallyposition the upper surface of fitting 181 on which the spring 175 pushesthe collar 186. This positioning of the fitting 181 and, hence, thecollar resting thereon, adjustably confines the spring 175 between thefirst and second support surfaces thereby preloading the spring at apredetermined preload force.

[0056] The preload force maintains the electrode assembly 102 and tube170 in their fully extended position until a force is applied oppositelydirected to the preload force overcoming the same so as to displace theelectrode assembly 102 and tube 170 upwardly respective to the rod 123and axially into the bore 129 against spring 175. When the electrodeassembly 102 is displace upwardly along the axis 150, the lower leg 162of the conductor 159 moves closer to the upper leg of the conductor 157.

[0057] To achieve a welding operation, a welding stroke begins in theposition shown in FIGS. 8 and 9. More specifically, the rod 123 isretracted into the drive cylinder 107 and the electrode assembly 102 isfully extended from the rod 123. The rod 123 drives the electrodeassembly 102 into contact with a workpiece after the rod travels a firstdistance which is less than the entire piston and rod travel distance.The rod 123 continues to travel a further short distance while theelectrode assembly 102 and tube 170 are spatially fixed due to theelectrode contacting the workpiece in alignment with the projection 51so as to compress the spring 175 by the same short distance, i.e., thewasher 176 moves closer to collar 186. The further compressed spring 175produces a second spring force, which force biases the electrodeassembly 102 in a downwardly direction toward the workpiece and providesthe force to the interface of the projection 51 and the adjacent sheetin contact therewith. Due to the further spring compression, the secondforce is greater than the first spring force, and in a preferredembodiment, the second spring force is reduced to about 200 lbs. whichdiffers from conventional forces applied to projections duringconventional projection welding operations which can be about 400 lbs.It is desirable to reduce the force applied to an aluminum projection sothat the current applied to the projection can likewise be reduced. Itis also desirable to preload the spring 175 to such an extent to preventwide variations in spring force initially transmitted to the electrodeassembly 102 in its fully extended position so as to avoid damage to theworkpieces and achieve a repeatable consistent projection weld.

[0058] After the electrode 140 contacts a workpiece and the rod 123continues to travel downwardly during the welding stroke, the fitting181, which is fixed to the rod 123, slides downwardly with the bushing184 sliding on the tube 170 away from the collar 186. The third boreportion 134 has a diameter at least slightly greater than the outerdiameter of the collar 186 so that the third bore portion can movedownwardly noncontactingly around the collar. The separation distancebetween the fitting 181 and collar 186 is equal to the maximum distancethat the electrode can travel to follow the collapse of the projection51, specifically the projection wall 55, into a weld nugget or jointfixing the two sheets together. It is desirable that the force createdby the spring 175 acting on the collapsing projection be reduced so thatless distortion, or read-through of the sheets is created and a lowerwelding current can be used to heat the projection.

[0059] Once the rod 123 reaches its downwardmost point and the spring175 is fully compressed, an electrical current welding pulse is appliedto the shunt 158 and travels through conductor 159 to the electrodeassembly 102. The current pulse preferably has a duration less than orequal to one-half wave cycle of an alternating current applied to thewelding system. If necessary, additional pulses can be applied tosufficiently heat the projection 51 to cause its collapse. When thecurrent pulse travels through the conductor 159, it creates a force onthe conductor 159 which tends to effect straightening of the conductorand thus tends to force the legs of the C-shaped conductor 159 to moveaway from one another. Since the upper conductor leg is fixed to theflange 153 and hence the stationary housing 107, the straightening forcethus acts on the lower conductor leg 162 as fixed to the electrodeassembly 102, which is in a nonfully extended position, and thus urgesleg 162 and electrode assembly downwardly toward the workpieces.Consequently, the conductor 159 assists the spring 175 to provide a fastresponse follow up to the collapse of the projection 51.

[0060] The welding gun assembly 101 thus provides an improved,repeatable follow up to the collapse of the projection 51 during aprojection welding operation. More specifically, the moving follow upstructure includes the electrode assembly 102, electrical conductor 159,and tube 170, all of which contribute to the improved follow up of theprojection 51 collapse. The mass of the electrode assembly 102 isreduced by a smaller electrode 140 being threadedly received in theadaptor block 141 which also has a reduced mass and a reduced thicknessclamp 145 securing the adaptor block 141 to the tube 170. The conductor159 has a C-shape with one leg being fixed on the housing bearing 122,and spacially fixed during a welding operation, and the other leg 162being fixed to the flange 147 of the clamp 145 transverse the axis 150of the welding gun assembly 101 for movement with the electrode assembly102. The conductor 159 has a tendency to straighten when a weldingcurrent pulse is applied thereto. Thus, another advantage is thepositioning of the conductor 159, which uses this tendency to supplementthe spring 175 forcing the electrode assembly's follow up of theprojection 51 collapse. A further advantage is the ends of the conductor159 being in close proximity to each other so as to minimize the mess ofthe conductor 159. Another advantage is the reduced mass of the hollowtube 170 which the spring 175 acts through to drive the electrodeassembly 102 downwardly following the collapse of the projection 51. Thewelding gun assembly 101 thus provided an improved follow up to theprojection 51 collapse.

[0061] In one embodiment of the invention, the sheet including theprojection thereon is an aluminum sheet, whereas the adjacent othermetal body may be a steel sheet. The projection is coined onto thealuminum sheet using the process described herein. The projection isbrought into contact with the steel sheet and welded according to theprocess and equipment described herein. Recent testing indicates that asufficient metallurgical securement between the aluminum and steelsheets can be created.

[0062] It is within the scope of this invention to coin nonhollowprojections on aluminum sheets, for example elongate ribs, which aresurrounded by the moat. These projections are columnar and are notpointed, i.e. they are rounded, at the apex thereof.

[0063] The projection described above and shown in the drawings has anannular and right cylindrical shape. It is within the scope of thisinvention to have other shapes such as nonright cylindrical, elliptical,etc.

[0064] While the above described embodiment of the welding systemdiscloses a transformer and controller for supplying the current pulse,it will be understood that any device capable of providing a weldingcurrent pulse is within the scope of this invention. Other conventionaltypes of such devices includes capacitor discharge and medium frequencydevices.

[0065] Although a particular preferred embodiment of the invention hasbeen disclosed in detail for illustrative purposes, it will berecognized that variations or modifications of the disclosedconstruction and apparatus, including the rearrangement of parts, liewithin the scope of the present invention.

The invention claimed is:
 1. A metal first body including an aluminumsheet portion and projection formed on said portion, said projectionbeing adapted to contact and projection weld said first body to anadjacent metal second body, said sheet portion having a thicknessextending between first and second planar side surfaces thereof, saidprojection comprising: an upright tubular wall extending generallyperpendicular to said first surface of said sheet portion; a firstrecess enclosed by said tubular wall and extending from a free endthereof toward said first surface; a second recess extending into saidsecond surface of said sheet, said second and first recesses beingsubstantially axially aligned; and a web separating said first andsecond recesses from one another.
 2. The projection according to claim1, wherein said wall extends annularly and said first recess is annular.3. The projection according to claim 1, wherein said second recess has awidth less than or equal to the width of said first recess so that saidsecond recess is positioned inwardly of said wall and said wall issupported by the full thickness of said sheet portion.
 4. The projectionaccording to claim 1, wherein said web has a thickness which is lessthan said sheet thickness.
 5. The projection according to claim 1,wherein said web has a first surface coplanar to said first surface andsaid second recess has a depth equal to or greater than half said sheetthickness.
 6. The projection according to claim 1, wherein saidprojection thickness equals 0.028 inch and said sheet thickness is inthe range of 0.030 to 0.035 inch.
 7. The projection according to claim1, wherein said sheet includes a depression in said first surfacesurroundingly adjacent said wall.
 8. The projection according to claim6, wherein said projection is a solid metal projection.
 9. A metal firstbody including an aluminum sheet portion and a projection coined in saidportion, said projection being adapted to contact and projection weldsaid first body to an adjacent metal second body, said sheet portionhaving a generally constant thickness extending between first and secondsubstantially parallel and planar side surfaces, said projectioncomprising: an upright wall extending generally perpendicular to saidfirst surface of said sheet portion; and a depression in said firstsurface adjacently surrounding said wall.
 10. A process for forming aprojection in an aluminum metal sheet so as to allow said sheet to beprojection welded to an adjacent metal body, comprising the steps of:positioning the sheet between first and second metal coining dies;pressing first and second dies respectively on opposed first and secondsurfaces of the sheet; squeezing the sheet between the two dies so as tocold flow metal radially and axially into a channel in the first die soas to create an upright hollow wall extending transverse to the firstsurface; and preventing the cold flowing material from flowing into thevolume interiorly of the wall to create a recess interiorly of the wallending at the first surface.
 11. The process according to claim 10,wherein the step of cold flowing includes flowing the metal from thesheet into the channel to form a solid wall that is perpendicular to thefirst surface.
 12. The process according to claim 10, wherein thesqueezing step includes cold flowing material from the second surface ofthe sheet to create a recess opening at the second surface and toprovide material movement to cause flow into the channel in the firstdie.
 13. The process according to claim 12, wherein the step ofpreventing includes axially aligning the recesses in the first andsecond surfaces.
 14. The process according to claim 12, wherein the stepof cold flowing includes preventing the material squeezed from therecess in the second surface from flowing laterally into the remainderof the sheet.
 15. A projection welding apparatus for creating projectionwelds between a metal sheet overlying an adjacent metal body, said sheethaving a projection extending toward the body, comprising: a stationaryhousing; a rod partially mounted in said housing and including a boretherein; an actuating device moving said rod relative to said housingfrom a start position to a contact position and an end position beyondsaid contact position; an electrode assembly mounted to a free end ofsaid rod, said electrode assembly including a hollow, lightweight tubereceived in said bore and extending axially from said rod and anelectrode adapted to contact one of the sheet and metal body when saidrod is in said contact and end positions; a spring unit engaging saidrod and tube so as to bias said tube axially outwardly of said rod; andan elongate, laminated electrical conductor including a first leg fixedto said housing and a second leg fixed to said electrode assembly, saidconductor being adapted to receive an electrical welding current at saidfirst end and transmit said current to said electrode assembly and theprojection on the sheet, said second leg extending transverse the axisand axial travel direction of said tube.
 16. The projection weldingapparatus according to claim 15, wherein said electrode assemblyincludes an adapter block and a clamp fixing said adapter block to afree end of said tube, said clamp includes a flange extending transversesaid tube and radially from said block, and said second leg being fixedto a surface of said clamp extending transverse to said tube.
 17. Theprojection welding apparatus according to claim 16, wherein saidelectrode assembly includes an electrode fixed to an end of said adapterblock remote said tube.
 18. The projection welding apparatus accordingto claim 15, wherein said tube includes electric insulation insulatingsaid tube from said rod.